1. Technical Field
The present invention relates to a method for determining whether a received signal of a positioning signal that is spread-modulated with a spread code is a multipath signal or not.
2. Related Art
As a positioning system that utilizes a positioning signal, a GPS (Global Positioning System) is widely known and used in a positioning device built in a portable telephone unit, car navigation apparatus or the like. In the GPS, positioning operation is carried out to calculate four parameter values, that is, three-dimensional coordinate values indicating the position of the machine itself and a clock error, on the basis of information such as the positions of plural GPS satellites and a pseudo-distance from each GPS satellite to the machine. Thus, the current position of the machine is measured.
A multipath is a principal element that generates an error in the positioning using the positioning signal. An environment where a multipath is generated is called multipath environment. A multipath environment refers to an environment where an indirect wave such as a reflected wave reflected on a building, ground or the like, a transmitted wave transmitted through an obstacle, or a diffracted wave diffracted from an obstacle is superimposed on a direct wave from a transmission source of the positioning signal (in the case of the GPS, the source is a GPS satellite), and these superimposed waves are received as a multipath signal. The indirect wave causes an error signal that makes it difficult to decode a code.
Various techniques are proposed in order to reduce the influence of this multipath. JP-A-2000-312163 discloses a technique of carrying out correlation operation between a received signal and each of plural replica codes of a C/A code having a different phase for every one chip, finding a phase range where the correlation value is expected to reach a peak, then carrying out correlation operation between the received signal and the plural replica codes having a different phase for every 0.1 chip with respect to the phase range, and thus detecting the code phase.
However, it is generally difficult to determine whether a received signal is a multipath signal or not. Therefore, generally, it is not determined whether a received signal is a multipath signal or not, and even if the signal is a multipath signal, a so-called defensive measure is employed to reduce the influence of the multipath signal on positioning operation. For example, the technique disclosed in JP-A-2000-312163 enables detection of a proper code phase even when a received signal is a multipath signal. Since such a defensive measure is employed, the price for this is paid in various forms such as increase in the quantity of operation and increase in the circuit scale.